1. Field of the Invention
The present invention relates to a communication apparatus, an integrated circuit and a communication method that are capable of easily detecting signals output from other communication apparatuses, which use different communication methods and are connected to a common transmission line, while avoiding interference between signals without performing relatively cumbersome modulation and other processes.
2. Description of Related Art
With the recent development of communication technology, PLC (Power Line Communication) has been gaining attention. PLC is a technology that performs multi-carrier communications among a plurality of terminal apparatuses, using power lines installed indoors as transmission lines, and utilizes an OFDM (Orthogonal Frequency Division Multiplexing) system (e.g., Japanese Patent Laid-Open Publication 2000-165304). OFDM is a modulation method for multi-carrier data transmission, by which a plurality of carriers are transmitted in a multiplex way on a frequency axis. OFDM uses an FFT (Fast Fourier Transform) or a DWT (Discrete Wavelet Transform) to narrow frequency intervals of multi-carriers and to closely space a plurality of carriers in such a way that they partially overlap and yet do not interfere with one another. OFDM thus enables broadband transmission by efficiently using a narrow frequency spectrum.
For multi-carrier communications, such as power line communications, a technology is proposed to suppress interference in such manner that a phase vector flattens time waveform levels to prevent occurrence significant peak. In this technology, when a time waveform has no significant peak, the phase of each sub-carrier is rotated using the phase vector of default. However, when the significant peak is detected, the phase vector is changed until a phase vector that generates no waveform peak is found, and the phase of each sub-carrier is thus rotated according to the changed phase vector (Denis J. G. Mestdagh and Paul M. P. Spruyt, “A Method to Reduce the Probability of Clipping in DMT-Based Transceivers”, IEEE Transactions on Communications, Vol. 44, No. 10, pp. 1234-1238, 1996). Such a technology for suppressing peaks is essential for reducing the design difficulty for a power amplifier for multi-carrier communications.
Usually, when the specifications of the same communication method are used, the specifications of communication apparatuses connected to each network are generally common even for a case where different logical networks are formed using a network key, or the like. This way, the communication apparatuses can detect (carrier sense) signals transmitted between different networks, on a physical layer level of the communication apparatuses, and it is possible to prevent interference between signals using a CSMA (Carrier Sense Multiple Access), thus enabling smooth communication even for relatively closely located different networks.
However, different manufacturers may use different specifications for a communication method such as a communication protocol, a modulation scheme and a frequency band. Such communication technology is highly likely to be used in an environment where a plurality of types of communication methods are mixed in the same location. For instance, users (communication apparatus users) in collective housing such as an apartment or a condominium do not necessarily use communication apparatuses (e.g., modems) of the same manufacturer. In this case, a plurality of types of communication apparatuses independently made by a plurality of manufacturers may be simultaneously connected to a common power line.
When the a plurality of types of communication apparatuses are connected to the common power line, a communication apparatus cannot demodulate a signal transmitted from a different communication apparatus using a different type of communication method. Therefore, such a signal is acknowledged merely as noise. Accordingly, although the plurality of types of communication apparatuses use the same frequency band, even the existence of other communication apparatuses is not acknowledged. This causes interference between signals transmitted from the plurality of types of communication apparatuses, thereby causing communication errors. In other words, the plurality of types of communication apparatuses sometimes cannot coexist on the common power line.
On the other hand, when each communication apparatus is set up to perform modulation, signals transmitted from other communication apparatuses can be differentiated. However, modulation processes performed to allow the plurality of types of communication apparatuses to coexist have an adverse effect of increasing the workload.